The present invention relates to a transfer belt for an electrophotographic apparatus and a method of manufacturing the same, particularly, to a transfer belt used in place of a transfer roll or a transfer drum in an electrophotographic apparatus, particularly, a plain paper copying machine (xerography) or a color printer, and to a method of manufacturing the same.
In a transfer system of toner from a photo-conductive drum included in a copying machine, a color printer, a facsimile, etc. using an electrophotographic system to a paper sheet, it was customary to use a transfer roll made of a sponge rubber having a semi-conductivity. In preparing the conventional transfer roll, an electrically conductive powder is added to an ethylene-propylene rubber to impart a semi-conductivity having an electrical resistivity of 10.sup.8 to 10.sup.9 .OMEGA..multidot.cm to the rubber, followed by forming the rubber into a sponge roll having an Asker C type hardness of about 30.degree. (Japan Hardcopy '91, p27).
In a transfer system using the transfer roll, the roll has such a small diameter as about 15 mm. Therefore, it is impossible to secure a wide nip width with the photoconductive drum, resulting in failure to achieve a high copying speed. In a color copying machine or a color printer, the copying speed is only about 10 sheets/minute.
It is conceivable to widen the nip width by increasing the diameter of the transfer roll. In this case, it is certainly possible to increase the copying speed. However, the apparatus itself is rendered bulky, making it impractical to increase the diameter of the transfer roll. Under the circumstances, it is proposed to widen the nip width by using a transfer belt in place of the transfer roll ("Electrography" by The Society of Electrophotography of Japan 30(3), 54(1991)).
FIG. 8 shows the concept of a copying machine. As shown in the drawing, a transfer roll 2, a developing roll 3 and a charging roll 4 for charging a photo-sensitive drum 1 are arranged in the vicinity of the photoconductive drum 1. Also, a fuser roller 5 is arranged downstream of the photoconductive drum 1. In the copying machine of the construction shown in FIG. 8, a toner image is formed on the photoconductive drum 1 by the developing roll 3 and, then, transferred onto a paper sheet 6 by the transfer roll 2. Further, the toner image transferred onto the paper sheet 6 is heated by the fuser roller 5 so as to be melted and fixed.
FIG. 9 shows the concept exemplifying a transfer belt system. The members of the system common with FIGS. 8 and 9 are denoted by the same reference numerals so as to omit the explanation thereof in the following description. As shown in FIG. 9, a corona transfer device 7 and a charging device 8 are arranged close to and slightly apart from the photoconductive drum 1. A transfer belt 10, which is driven by driving rolls 9a, 9b and supported by a supporting roll 9c, is stretched so as to be transferred between the photo-conductive drum 1 and the corona transfer device 7. As apparent from FIG. 9, the nip width between the photoconductive drum 1 and the transfer belt 10 can be widened.
The transfer roll and the transfer belt are required to exhibit a stable electrical resistivity, i.e., a semi-conductivity of 10.sup.7 to 10.sup.13 .OMEGA..multidot.cm. Particularly, these roll and belt are required to be low in fluctuation of the electrical resistivity under various environments. To be more specific, the transfer roll and the transfer belt are required to exhibit less than an exponent of fluctuation in the resistivity under various conditions such as a high temperature-high relative humidity RH of 30.degree. C. and 80%, a low temperature-low relative humidity RH of 23.degree. C. and 5%, or an ordinary temperature-ordinary relative humidity RH of 23.degree. C. and 55%. The reasons for the requirement of the stable resistivity are that, if the resistivity is unduly low, the voltage is lowered, resulting in failure for the roll or belt to be charged, and that, if the resistivity is unduly high, current is unlikely to flow through the roll or belt, leading to a low charging potential.
What should also be noted is that ozone is generated within the copying machine or the printer. Naturally, it is important for the transfer roll or the transfer belt to exhibit a high resistance to ozone and a reasonable heat resistance and not to contaminate the photoconductive drum. In addition to these requirements, the transfer belt is required not to run zigzag during operation of the copying machine, to be low in elongation relative to a predetermined tension, to exhibit an excellent dimensional stability, and not to bear a permanent set over a long period of time. In general, the transfer belt is said to receive a tension of 2 to 3 Kgf/cm and to exhibit an elongation of about 5%. In other words, the belt is elongated by about 5% during operation of the copying machine. Further, the belt surface is required to readily release the toner and to have a small friction coefficient.
A transfer belt prepared by extruding a polyurethane rubber in a cylindrical form, followed by vulcanizing the extrudate and subsequently polishing the surface of the extrudate and, then, coating the surface of the extrudate with a fluorine resin is disclosed in "Electrography" by The society of Electrophotography of Japan 33(1), 43(1994)".
However, a reinforcing layer is not incorporated in the polyurethane rubber transfer belt noted above, with the result that relaxation of stress takes place if tensity of the transfer belt is continued. In this case, a permanent set is generated so as to give rise to loosening of the belt. It follows that the transfer belt fails to be rotated as desired so as to distort the toner image. In other words, the transfer belt fails to perform its function.
An additional defect inherent in the polyurethane rubber transfer belt is that the rubber tends to be markedly affected by the environment, particularly, by the humidity. In some cases, depolymerization of the polyurethane rubber is brought about by the humidity, with the result that the hardness of the transfer belt is markedly lowered. In addition, the resistivity of the transfer belt under an environment of a low humidity is more than 10 times as high as that under an environment of a high humidity, which is a decisive defect. Needless to say, the resistivity of the polyurethane rubber is markedly lowered with increase in humidity of the environment.
Further, the polyurethane rubber is poor in resistance to ozone and heat generated within the copying machine or the printer.
A method of manufacturing an electrically conductive rubber belt is also disclosed in Japanese Patent Disclosure (Kokai) No. 4-99446. It is disclosed that a roll coater is arranged on a rotating shaft, and a tank housing a solid rubber paste mixed with a conductivity-imparting agent is moved from one side so as to coat the shaft with the solid rubber paste and to crosslink the rubber. The method disclosed in this prior art is directed to formation of a solid rubber single layer and is intended to prevent non-uniformity of the electrical resistivity over the entire rubber layer which may be derived from the heat history in the forming or vulcanizing step and from difference in pressure applied to the rubber layer.